(1) Technical Field
The present invention relates to a semiconductor laser module and a method of manufacturing the same.
(2) Description of Related Art
Up to now, a semiconductor laser device that outputs laser light obtained by combining two laser lights (or through polarization synthesis of two laser lights) has been disclosed in, for example, U.S. Pat. No. 5,589,684 B or JP 2000-31575 A.
FIG. 15 is an explanatory diagram showing a conventional semiconductor laser apparatus as disclosed in U.S. Pat. No. 5,589,684.
As shown in FIG. 15, the conventional semiconductor laser apparatus comprises a first semiconductor laser device 100 and a second semiconductor laser device 101 emitting laser beams of identical wavelengths in mutually orthogonal directions; a first collimating lens 102 configured to collimate the laser beam emitted from the first semiconductor laser device 100; a second collimating lens 103 configured to collimate the laser beam emitted from the second semiconductor laser device 101; a polarization-synthesizing device (i.e. cube beam splitter) 104 configured to polarization-synthesize the laser beams that were collimated by the first collimating lens 102 and the second collimating lens 103; a convergent lens 105 configured to converge the laser beams polarization-synthesized by the polarization-synthesizing device 104; and an optical fiber 106 for receiving the laser beams converged by the convergent lens 105 and letting the laser beams travel outside.
In the conventional semiconductor laser apparatus, the laser beams are emitted from the first semiconductor laser device 100 and the second semiconductor laser device 101 in mutually vertical directions and are polarization-synthesized by the polarization-synthesizing device 104 to obtain a laser beam of reduced DOP (Degree Of Polarization) from the optical fiber 106. (This technology will hereinafter be called a prior art 1.)
Further, Japanese Patent Application Laid-open No. 2000-31575 discloses a semiconductor laser module including a thermoelectric cooler; a first and a second semiconductor laser devices mounted on the thermoelectric cooler; two lenses each for collimating the first and second laser beams emitted from the first and second semiconductor laser devices; a polarization-synthesizer for synthesizing the first and second laser beams; and an optical fiber for receiving the laser beams emerging from the polarization synthesizer and letting the laser beams travel outside (see FIG. 5 or FIG. 10 of Japanese Patent Application Laid-open No. 2000-31575). Moreover, the first and second semiconductor laser devices are formed in an LD array, in which the laser diodes are arrayed at a pitch between their light-emitting centers (hereinafter referred to as inter-emission-center pitch) of 500 μm. Further, the first and second convergent lenses are formed in a lens array such as a ball lens array or a Fresnel lens array. (This technology will hereinafter be called a prior art 2.)
Further, the applicant of the present invention has proposed a semiconductor laser module in which two laser beams emitted from two light-emitting stripes (hereinafter referred to simply as stripes) formed in a single semiconductor laser device are polarization-synthesized and received by an optical fiber. (See Japanese patent application No. 2001-383840, for example. This technology will hereinafter be called a related art.)
FIG. 2 is an explanatory diagram schematically showing a configuration of the semiconductor laser module of the related art.
As shown in FIG. 2, the semiconductor laser module of the related art includes a single semiconductor laser device 2 having a first stripe 9 and a second stripe 10 formed in parallel to each other interposed therebetween and emitting a first laser beam K1 and a second laser beam K2 from a front end face (i.e. an end face on right-hand side in FIG. 2) of the first stripe 9 and the second stripe 10 respectively; a first lens 4 positioned so that the first laser beam K1 and the second laser beam K2 are incident therealong and configured to separate the first laser beam K1 and the second laser beam K2 in the direction in which the first and second stripes 9, 10 are arrayed; a half-wave plate 6 (a polarization rotating means) configured to rotate a polarization direction of at least one of the first and second laser beam K1, K2 (i.e. the first laser beam K1 in FIG. 2) by a predetermined angle (by 90 degrees, for example); a PBC (Polarization Beam Combiner) 7 configured to optically synthesize therealong the first laser beam K1 and the second laser beam K2; and an optical fiber 8 optically coupled to the synthesized laser beams emerging from the PBC 7 and letting the synthesized beams to travel outside.
In addition, a prism 5 is disposed between the first lens 4 and the half-wave plate 6 so that the first laser beam K1 and the second laser beam K2 are incident thereon and output therefrom along their respective optical axes parallel to each other. This prism 5 includes an incident surface 5a positioned so that the first laser beam K1 and the second laser beam K2 are incident and vertically disposed to the optical axis of optical fiber 8, and an exit surface 5b inclined at a predetermined angle from the incident surface 5a. Further, a second lens 16 is disposed between the birefringence element 7 and the optical fiber 8 in order to optically couple the first and second laser beams K1, K2 optically combined by the PBC 7 to the optical fiber 8 which is supported by a ferrule 23.
The first laser beam K1 and the second laser beam K2 emitted respectively from the front end face 2a of the first stripe 9 and the second stripe 10 of the semiconductor laser device 2 travel through the first lens 4, intersect and separate until the separation between the two beams is enough, before entering the prism 5.
During propagation through the prism 5, the first laser beam K1 and the second laser beam K2 are made parallel to each other, and are emitted from the prism 5. The first laser beam K1 then enters the half-wave plate 6, where its polarization direction is rotated by 90 degrees, and then enters a first input port 7a of the PBC 7, while the second laser beam K2 enters a second input port 7b of the PBC 7.
The first laser beam K1 incident on the first input port 7a and the second laser beam K2 incident on the second input port 7b are optically coupled along the PBC 7, and output from an output port 7c. 
The laser beams emerging from the output port 7c of the PBC 7 are then converged by the second lens 16, enter an end face of the optical fiber 8 supported by the ferrule 23, and propagate to outside.
In Prior Art 1, Prior Art 2, and Related Art, a semiconductor laser device is fixed to the top of a base by solder or the like, and YAG laser welding or the like is used to fix a lens to the top of the base. If the thickness of solder is not uniform or the lens is YAG-welded at a wrong position in this fixing process, for example, it makes the plane defined by optical axes of two laser lights that are emitted from the one or two semiconductor laser devices unparallel to a mount surface (principal surface) of the base to which the semiconductor laser device(s) is (are) to be fixed. In this case, the two laser lights have different spot positions at the output port of the light combining element, and the exit optical axes do not coincide with each other, leading to a failure in combining the light beams appropriately.
In the case of polarization synthesis, a problem is that the intensity of synthesized light after polarization synthesis is lowered by polarization split, which takes place when laser light emitted from the semiconductor laser device enters the light combining element (polarized wave synthesizing element) while polarized in an inappropriate polarization direction.
A specific description is given on the problems referring to FIGS. 16. FIG. 16(A) is a perspective view schematically showing the structure of the semiconductor laser module of Related Art illustrated in FIG. 2. FIG. 16(B) is a diagram showing positions and polarization directions of the laser lights K1 and K2 on the incident surface 5a of the prism 5. FIG. 16(C) is a diagram showing positions and polarization directions of the laser lights K1 and K2 in the input ports 7a and 7b of the PBC 7.
As shown in FIGS. 16(A) and 16(B), when the two laser lights K1 and K2 enter the prism 5 while a plane P, which is defined by the optical axes of the two laser lights K1 and K2, is not parallel to a mount surface Q of a base to which the semiconductor laser device 2 and others are fixed, the laser lights K1 and K2 on the exit surfaces 5b have different refraction angles. Accordingly, the optical axes of the two laser lights that have exited the prism are no longer on the same plane. After the two laser lights K1 and K2 exit the exit surfaces 5b, the laser light K1 of the two laser lights K1 and K2 enters the input port 7a of the light combining element (PBC) 7 with its polarization direction rotated by 90° by the half-wave plate 6 whereas the laser light K2 enters the input port 7b of the light combining element 7 as it is. The laser lights are then subjected to polarization synthesis in the light combining element 7 and exit from the output port 7c. As a result, the laser lights K1 and K2 in the output port 7c are offset from each other in the Y-axis direction by δ and the exit optical axes of the laser lights K1 and K2 do not coincide with each other.
In the case where a polarized wave synthesizing element is used as a light combining element as in the semiconductor laser module of Related Art shown in FIG. 2, the two laser lights K1 and K2 enter the input ports 7a and 7b, respectively, of the light combining element (PBC) 7 with their polarization directions shifted from a given direction as shown in FIG. 16(C). This causes polarization split in which the laser light K1 is split into an ordinary ray K1n and an extraordinary ray K1a, and the laser light K2 is split into an ordinary ray K2n and an extraordinary ray K2a (see FIG. 16(A)). As a result, the intensity of laser light that exits from the output port 7c is lowered.
Accordingly, synthesized light coupled to the optical fiber 8 does not have desired intensity and degree of polarization in some cases.